The present invention relates to a thin high-strength article of spheroidal graphite cast iron and a method of producing it.
In the production of spheroidal graphite cast iron having a ferrite matrix, cast iron products are usually left to stand in the air after removal from molds so that they are cooled to low temperatures such as room temperature, and they are heated again to temperatures higher than their A.sub.3 transformation points, particularly 850.degree.-950.degree. C. to conduct the ferritization of pearlite contained in their matrices. When this heat treatment is conducted on thin spheroidal graphite cast iron products, primarily precipitated graphite particles hereinafter referred to as "primary graphite particles" are diffused in the matrices, leaving fine gaps around their graphite particles. As a result, the thin spheroidal graphite cast iron products inevitably have reduced mechanical properties particularly fatigue strength.
In addition, since the spheroidal graphite cast iron products are heated to a high temperature after cooled to room temperature, a large amount of thermal energy is consumed, meaning that this process is economically disadvantageous.
Japanese Patent Laid-Open No. 57-28669 discloses a method of producing a thin as-cast spheroidal graphite cast iron product. In this method, a spheroidal graphite cast iron product having portions of different thicknesses is cooled such that every portion is cooled at a cooling speed of 13 .degree. C./min or more, so that a matrix structure containing 50.degree.-90.degree. of pearlite can be stably obtained in an as-cast state. However, this method fails to provide high-strength spheroidal graphite cast iron products having matrices substantially consisting of ferrite and free from fine gaps around graphite particles, thereby showing excellent mechanical properties.